This invention relates to the manufacture of polymers by anionic polymerization of monomers, especially conjugated dienes and/or vinyl aromatic hydrocarbons, in a hydrocarbon solvent. More particularly, this invention relates to an improvement in such a process whereby the throughput of the manufacturing system is increased by reducing the viscosity of the polymer cement (the solution of the anionic polymer in the hydrocarbon solvent).
Anionic polymers, including polymers of conjugated dienes and/or vinyl aromatic hydrocarbons, have been produced by numerous methods. However, anionic polymerization of such or other monomers in the presence of an anionic polymerization initiator is the most widely used commercial process. The polymerization is carried out in an inert solvent such as hexane, cyclohexane, or toluene and the polymerization initiator is commonly an organo alkali metal compound, especially alkyl lithium compounds. The solvent used is almost always a non-polar hydrocarbon because such solvents are much better solvents for the polymers of such monomers, especially conjugated diene polymers or blocks when they form a part of block copolymers.
As the polymer is created from the monomers, a solution of the polymer forms in the inert hydrocarbon solvent. This solution is called the polymer cement. These polymerizations may be carried out at a variety of solids contents and it is reasonably obvious that if the process can be run at high solids content, the manufacturing cost will be decreased because the cost of solvent will be decreased and more polymer can be produced in a given amount of time.
Unfortunately, with polymer cements of anionic polymers, one of the most significant rate limiting aspects is the viscosity of the polymer cement. This is especially true in the manufacture of block copolymers of conjugated dienes such as butadiene or isoprene and vinyl aromatic hydrocarbons such as styrene.
Solutions of living anionic polymers, living polymer cements, tend to be higher in viscosity than their terminated analogs, terminated polymer cements. The higher viscosity of living polymer cements in polymerization tends to limit the production capacity of equipment used to make these products. Higher concentrations of terminated polymer solutions could be pumped and mixed with the existing equipment but polymerization at these higher concentrations is not possible due to the prohibitively high viscosities of the living polymer solutions. Production rates are limited by the viscosity of the living anionic polymer solutions in polymerization since the polymer chain must be kept xe2x80x9clivingxe2x80x9d, i.e., not terminated, until the desired molecular weight is achieved.
The present invention is an improvement upon the known method of anionically polymerizing monomers by contacting the monomers with an anionic polymerization initiator which is an organo-substituted alkali metal compound. The improvement comprises decreasing the viscosity of the polymer cement by adding at least 0.01 equivalent of a metal alkyl compound per equivalent of alkali metal initiator if the metal alkyl is added before or at the beginning of polymerization. If the metal alkyl is added during the polymerization or after but before the living polymer is terminated, then at least 0.01 equivalent of the metal alkyl compound per equivalent of living polymer chain ends (i.e., styryl-lithium or dienyl-lithium moiety) should be used. Preferably, in both cases, from 0.01 to 1.5 equivalents is used and most preferably, 0.01 to 1.0 equivalents.
The metal alkyl is preferably added during the polymerization but it can be added before polymerization begins. It can also be added subsequent to polymerization before termination if desired. The alkyl groups of the metal alkyl are chosen such that they do not exchange with the organo substituents of the alkali metal, which can be the living polymer chain ends or the organo substituents of the initiator. To avoid this undesired exchange reaction, the alkyl groups of the metal alkyl compound are selected to be more basic and/or less bulky or both than the organo substituents of the alkali metal compound. The organo substituents of the alkali metal compound are aliphatic, cycloaliphatic, aromatic, or alkyl-substituted aromatic and include multi-functional initiators such as the sec-butyl lithium adduct of diisopropenyl. In a preferred embodiment of the invention, the organo-substituted alkali metal species at the time of the addition of the metal alkyl is a styryl-lithium or dienyl-lithium moiety. The preferred metal alkyl for use herein is triethyl aluminum.